Nearly all distribution cable now being installed in domestic telephone systems is buried beneath the ground. This represents a large investment in cable and the reliability of that cable as measured by maintenance costs and service continuity is an important factor. Most of the cable is waterproofed, because the most antagonistic environmental factor for buried cable is water.
Attempts to waterproof buried cable began nearly one hundred years ago and were unsuccessful in a practical sense until the introduction of plastic insulated cable (PIC) during the 1950's. Specially sheathed cables, with dual plastic coatings encasing an aluminum sheath have been used successfully and are still being buried in dry environments. Pressurized cable also contends successfully with water problems. However, both of these approaches are deficient, the former leaves the cable vulnerable and the latter is expensive to maintain. Since 1970, large quantities of cable have been filled with waterproofing compounds. This approach followed the recognition that in PIC cable the localized intrusion of water into the cable sheath is not in itself a serious problem. Disruption or deterioration of service occurs when long lengths of cable become flooded. Flooding occurs because water that penetrates into a localized opening in the cable sheath is free to channel as far as gravity allows, often hundreds of feet. Not only does this upset the capacitance balance of the transmission lines, but it introduces more potential corrosion sites in proportion to the length of wire that is wetted. Corrosion typically occurs slowly, but the useful life of water soaked wires is obviously shorter than that of dry wires.
The solution that has been widely adopted is to fill the voids in the cable with an insoluble filling material that simply plugs the cable to channeling water. However, though the physical function of the cable filling material is straightforward, the choice of the material is not. Among the many considerations that are important for materials used in this application are the hydrophobic nature of the material, stability on aging, low temperature properties, flow characteristics at elevated temperatures, processing characteristics, handling characteristics, dielectric properties, toxicity and cost.
Materials that satisfy most of these criteria, and which have been used widely in this application, are described in U.S. Pat. Nos. 3,607,487 and 3,717,716 issued Sept. 21, 1971 and Feb. 20, 1973 respectively. These materials are essentially a petroleum jelly, mixed with a polymer, usually polyethylene, to impart consistency and prevent flowing at warm temperatures.
Similar hydrophobic filling materials have been proposed for filling splice closures. For example, U.S. Pat. No. 3,879,575 issued Apr. 22, 1975 describes a mixture of a low viscosity oil gelled by a styrene-isoprene-styrene copolymer, again with polyethylene added to impart consistency and reduce slump.